The television broadcast industry has always depended upon sampling TV viewers to indicate to a particular advertiser the network's individual share of the potential audience, Various systems have been developed to automatically measure viewership but with mixed results. Historically, systems which depend upon the network affiliates' cooperation have usually been compromised because these systems interfered with signal handling equipment or the system itself became a tool which the network could use to detect program substitutions made by the affiliate.
Classic approaches to the task of audience measurement have been either to modify the TV set, add external circuits such as tuners, or measure the TV set's local oscillator frequency, all of these techniques being indirect methods of finding out what a viewer is watching. These indirect methods have practical problems since more and more television is distributed by cable TV systems which often indiscriminately reassign different channels to the network stations.
It is known to conduct surveys of television audiences to determine the popularity of television programs. Various sytems have been developed for automatically determining which station is being viewed to reduce interaction between the person conducting the survey and the television-viewer.
U.S. Pat. No. 2,903,508 to Hathaway teaches a survey system wherein the horizontal and vertical synchronization signals are subjected to a cyclical phase shift to "tag" the broadcast signal. A magnetic induction pickup receives the synchronization signals emitted by the television receiver and processes these signals to detect the cyclical phase shift. A second receiver is tuned to a known station and the cyclical phase shift is monitored. When the cyclical phase shifts match, it is concluded that the television set being monitored is tuned to the same station as that of the known television receiver.
U.S. Pat. No. 3,130,265 to Leonard teaches a system for determining the channel to which a television receiver is tuned by also relying upon detection of the phase of synchronization (sync) pulses. In this system, transmitters are controlled so that the sync pulses of each transmitter are out of phase by a known amount with respect to the sync pulses of all other transmitters. This sytem requires that the conductor of the survey have control over the broadcast transmitters.
Systems such as those shown in U.S. Pat. Nos. 3,312,900 to Jaffe and 4,577,220 to Laxton et al detect the frequency to which a local oscillator of a receiver is tuned to determine the channel being viewed.
In a system shown in U.S. Pat. No. 3,806,805 to Wall, a television receiver imposes a load variation pattern on the main power supply line which is representative of the channel to which the receiver is tuned. An audience measuring system is responsive to variations in the main power supply line to identify the station.
U.S. Pat. No. 3,372,233 to Currey teaches a monitoring system wherein the sync signal of a monitored receiver is combined with the sync signal of a receiver tuned to a known station. The phase relationship of these two sync signals indicates whether the monitored receiver is tuned to the known station.
U.S. Pat. No. 4,847,685 to Gall teaches a system for automatically surveying television viewership by detecting both the horizontal and vertical sync pulses and determining their relative phase from a reference counter driven from another TV tuner. The difference in phase corrected by distance the signal travels is an indication of which channel is being viewed.
U.S. Pat. No. 5,019,899 to Boles et al is an electronic data encoding and recognition system which processes each frame of a TV program to form a digital signature of each frame. These digital signatures are compared with a database of signatures to determine the identity of a program in TV transmission.
U.S. Pat. No. 4,807,031 to Broughton et al teaches a system wherein control data are encoded on only a portion of a selected sequence of video test image fields. This sytem is unsatisfactory because it requires an optical sensor to be attached on the viewer's TV screen which blocks the image, is sensitive to variations in brightness, requires selected video content, introduces visual distortion, is far too slow to identify programs at a rate sufficient for the industry's needs and does not permit unobtrusive sensing of the television set in the householder's home.
An article entitled "A Novel TV Add-On Communication System", IEEE Transactions On Broadcast And Television Receivers, Vol. BTR-19, No. 4, pp. 225-230 by Patrick T. King describes a television modulation system in which added information is sent through a television channel along with the standard video signal. This paper describes modulating lines of a TV picture but it is clear that the paper does not utilize line modulation in the method described herein.
None of the known TV program identifying systems is satisfactory for present day industry needs.